Seizures in 5.5 month old hen

I have a young black copper maran who's been bullied out of food and water from by our 3 other chickens. We bought them together, from the same breeder, about 2 months ago. They were all the same size at that time. About 3 weeks ago I noticed that she wasn't growing as quickly as the other hens but she was was a very happy chicken. We gave her supplemental foods (scrambled eggs, berries, dark leafy greens) away from the flock during those three weeks in an attempt to help her keep up. About a week ago she started to decline: sluggish (but moving around looking for bugs), eyes half open.

I removed her from the coop about 3 days ago when she started isolating herself, nearly completely listless, very sleepy, stopped eating & drinking, pale comb, dark green water poop. She's been inside our home since.

I've been able to get her to eat and drink very little - water with probiotics and layer feed mash with yogurt. I gave her dewormer, but I strongly believe the root cause is dehydration and starvation.

She has shown no improvements. She had a seizure around 1:30am this morning, another at some point in the night, and then again around 11:15am.

I'm really not sure what to do. I don't want her to suffer, but I also don't want to make the decision to euthanize without knowing I did everything I could to help her heal.

I also have a hen that is almost 4 years old that fits the description of your hen, except for seizure. Marek's can cause neurologic symptoms that may look like seizure.
I have had my hen in the house since December 2016. She improves, eats well and seems fairly normal, then relapses and appears to be blind, won't eat, loses weight, etc. Right now she has "recovered" for the second time and I have her outside in her own small coop. She has always been kind of sickly and also at the bottom of the pecking order.

I suspect she has Marek's and am struggling with the decision of whether to have her euthanized.

look for bloody stool (coccidiosis ).... I would give her corrid and the others also...
you add it to their water...
She now may also be depressed to be away from the others....
poly vi sol without iron is good way to get vitamins in her...
maple syrup is also a great pick me up.... just a half a dropper full on the end of her
tongue.... Wishing you the best...! Be patient you might be able to help her get better... =)

who's been bullied out of food and water About a week ago she started to decline: sluggish (but moving around looking for bugs), eyes half open.completely listless, very sleepy, stopped eating & drinking, pale comb, dark green water poop.water with probiotics and layer feed mash with yogurt.dehydration and starvation.

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Has she started laying eggs?
It's so hard to know what's going on, but the dark green watery poop would most likely be an indication of dehydration and starvation since she was being kept from food.

Treating her for Cocci may not be a bad idea, but getting a fecal float for confirmation would be best. Reason being, she needs vitamins and these generally are not given along with Corid.

Offer her some poultry vitamins in her water - plenty of hydration. Offer her some wet chick starter or a flock raiser formula feed if you have it. Egg, tuna, beef liver, etc. can be given to boost nutrients and protein as well. Layer mash is for actively laying hens, so if she is not laying eggs, then go with one of the above feeds.

If she has been bullied severely, she may have suffered some type of head injury/trauma and this is the cause of the seizures along with not enough nutrition.